1. Field of the Invention
The present invention relates to a fail-safe scan circuit for sequentially and periodically driving and scanning a plurality of objects and to a fail-safe multibeam sensor.
2. Related Art of the Invention
U.S. Pat. No. 4,309,696 discloses a multibeam sensor used for securing the safety of an operator of a machine such as a press.
The press has a danger zone in which an operator must not be present during the operation of the press and a safe zone in which the operator is safe. At a boundary between the danger and safe zones, many light emitters and photodetectors are oppositely arranged. The light emitters emit beams toward the photodetectors. If the operator blocks at least one of the beams, a corresponding one of the photodetectors will not provide an output signal. Then, it is determined that the operator is in the danger zone, and the press is stopped.
The light emitters and photodetectors are vertically positioned at intervals of about, for example, two centimeters. Since any beam from the emitters more or less diffuses, the beam may be received not only by a corresponding one of the photodetectors but also by adjacent ones. In this case, several photodetectors will simultaneously provide signals in response to the beam. To avoid this, the emitters may emit beams at different frequencies, and each photodetector may have a filter that extracts only a signal of corresponding frequency. This technique, however, involves as many frequencies and filters as the light emitters and photodetectors.
To avoid an increase in the numbers of these parts, a technique has been proposed to employ a scan circuit that sequentially and periodically drives pairs of the light emitters and photodetectors one after another. Conventional scan circuits, however, are incapable of detecting abnormalities such as simultaneous emission of beams due to simultaneously generated scan signals.
Accordingly, the inventors of the present invention have proposed PCT/JP92/00631 disclosing a fail-safe scan circuit for periodically providing scan signals to objective devices. This circuit generates a signal of logical value 1 under a normal state, and upon detecting an abnormality, stops the signal.
This fail-safe scan circuit has a counter circuit for periodically counting as many pulses of a clock signal as the objective devices; a scan signal generator for consecutively generating as many scan signals as the objective devices in response to count signals provided by the counter circuit; and a fail-safe failure detector.
The failure detector provides a signal of logical value 1 representing a high energy state if the scan signals are normal and a signal of logical value 0 representing a low energy state to inform of an abnormality. The abnormality may be simultaneous generation of some of the scan signals, or a lock of at least one of the scan signals in a single scan period.
The signal of logical value 1 provided by the failure detector is used as an enable signal for starting a machine such as a press. Upon detecting an abnormality in the scan circuit, the failure detector provides the signal of logical value 0 to stop the press.